1. Field of the Invention
The present invention relates to a semiconductor memory device, and particularly a semiconductor memory device allowing reduction in number of external pins which are employed for external transmission of signals.
2. Description of the Background Art
A semiconductor memory device performs input/output of data signals as well as storage thereof based on externally supplied control signals and address signals. These signals are transmitted via pins arranged on a chip.
FIG. 26 shows a pin arrangement of a conventional semiconductor memory device 500.
A pin arrangement of a non-synchronous SRAM (Static Random Access Memory) is shown in FIG. 26 as an example of a pin arrangement of a conventional semiconductor memory device.
Referring to FIG. 26, control pins 1-5 are employed for input of control signals such as a chip select signal, a write control input signal and an output enable signal. Address pins A0-A15 are provided for input of respective bits of an address signal. In the following description, each bit of the address signal may also be referred to as an "address bit". DQ1-DQ15 indicate DQ pins for input/output of the respective bits of an I/O data signal. In the following description, each bit of the I/O data signal may also be referred to as a "data bit".
In semiconductor memory device 500, the I/O data signal of 16 bits are externally read or written in response to the address signal of 16 bits.
Semiconductor memory device 500 further includes pins for receiving a ground potential VSS and a power supply voltage VCC, respectively. A pin indicated by "NC" is an unconnected pin.
In the conventional semiconductor memory device, as described above, the independent pins are provided for the signals of different functions, respectively.
However, capacities and functions of the memory devices have been improved. This results in increase in number of the bits of the address signal for selecting the memory cell and the bits of the I/O data signal which are simultaneously input/output, and also results in disadvantages such as increase of the control signals due to addition of control functions. Thereby, the device employs more pins, which increases a chip size of the device. Conversely, semiconductor memory devices such as an asynchronous SRAM have been employed in many devices such as portable telephones. For such employment, it is important to reduce a layout area.
With increase in pin number, the pins which operate simultaneously with each other increase in number, and therefore such a problem arises in the input buffer circuit, which receives data supplied through the pins, that the sum of consumed currents increases due to through-currents of transistors forming an input first stage.